1. The Field of the Invention
The present invention is related to the attachment of a boss to a composite pressure vessel, and more particularly to a method and device for securing a polar boss to a composite pressure vessel without relying on an adhesive placed between the boss and the vessel.
2. Technical Background
Composite pressure vessels are constructed by securing reinforcing fibers in a resin matrix. Composite construction provides vessels which are both lightweight and strong. Accordingly, composite vessels are used in a wide variety of applications to store fluids under pressure. Each composite vessel defines a pressure chamber which contains a pressurized fluid that is moved in a controlled manner in or out of the vessel.
For instance, pressurized oxygen is drawn from such vessels by fire fighters, scuba divers, and other people. Pressurized fluids for extinguishing fires are stored in such vessels in homes, public buildings, and many other locations. Compressed natural gas (CNG) is drawn from pressure vessels to fuel a variety of vehicles. Pressurized oxygen, acetylene, and other flammable gases are stored in such vessels for use by metal workers. Hospitals and dental offices store pressurized anesthetic gases and oxygen in pressure vessels, and research laboratories hold liquid nitrogen and other cryogenic fluids in such vessels.
Because the resin matrix of a composite vessel may crack during use, many composite vessels include a fluid-impermeable liner. Both metallic and non-metallic liners are used, but non-metallic liners are preferred because of their lighter weight. Such liners may be designed not merely to serve as fluid barriers but also to act as a mandrel during fiber winding when the vessel is fabricated using that method.
To permit controlled movement of fluids in or out of the pressure chamber, the vessel is typically configured with a pressure chamber orifice and a boss is fitted about the orifice. The boss is threaded or otherwise shaped for connection to nozzles, valves, gauges, tubes, and similar fixtures which direct and control fluid flow. Accordingly, the boss is formed of metal or another strong, rigid material.
The boss typically includes a cylindrical neck with a longitudinal passage that provides fluid communication between the pressure chamber and the environment outside the vessel. A longitudinal axis is defined within the neck substantially parallel to the passage. A flange is secured to one end of the neck. The flange, which is larger than the pressure chamber orifice, is secured to the liner of the pressure vessel to hold the boss in place. Movement of the flange with respect to the liner is preferably restricted in at least two ways.
First, translation of the boss should be prevented. That is, the boss should not be permitted to move along the central longitudinal axis of the neck such that it falls completely inside the pressure chamber. Nor should the boss be allowed to move in the opposite direction and separate itself from the vessel.
Second, rotation of the boss should be prevented. That is, the boss should not be permitted to rotate with respect to the liner about the central axis of the neck or about the pressure chamber orifice. Thus, when the vessel is held and an attempt is made to thread a fixture into the threaded boss, the boss should not thwart the attempt by rotating with respect to the liner.
The problem therefore arises of how to attach the boss securely to a non-metallic liner to prevent boss translation and rotation. One approach simply relies on the internal pressure in the vessel to attach the boss and liner. Although this pressure may suffice to create a seal between the boss and a non-metallic liner in some vessels, and may prevent boss translation, it generally does not prevent boss rotation.
Another approach positions the boss between two lips of a partially bifurcated liner. Such a liner includes two lip layers disposed about the circumference of the pressure chamber orifice. The boss flange is positioned in an annular recess between the lip layers and is encapsulated by the liner lips. No adhesive is used between the boss and the liner lips. Although this approach limits boss translation, it fails to substantially restrict boss rotation.
A different approach to securing the boss to the liner includes placing a layer of adhesive between the boss and the liner. The adhesive bonds the boss and the liner together, at least initially. Unfortunately, some adhesives deteriorate over time. Thus, the adhesive may not last the entire 15 years needed to span the service life of a CNG container, particularly if hydrothermal stresses or chemical contaminants are introduced into the vessel.
It is also difficult to bond metal bosses to certain thermoplastic materials such as nylons and polyolefins, because petroleum-based thermoplastic materials inherently have a so-called "lubricated" surface. The use of adhesives to bond metal to such materials typically involves elaborate surface preparation and treatment steps to ensure an adequate bond. The repeatability of adequate bonding also becomes operator sensitive and hence problematic in large volume manufacturing.
Moreover, using an adhesive layer complicates vessel manufacturing by adding adhesive application and bonding steps. Because the adhesive is an additional material to be purchased, stored prior to use, and incorporated into the vessel, using adhesive also increases the vessel's cost.
Adhesives are also of limited use in applications which subject the boss and liner to repeated or large torques relative to one another. In such applications the adhesive tends to shear apart. For instance, the adhesive may fail in response to torques produced by repeatedly threading fixtures in and out of the boss. Once the adhesive layer shears apart it is generally impractical to attempt repair, and the pressure vessel loses much of its usefulness.
From the foregoing, it can be seen that it would be an advancement in the art to provide a method and device for attaching a boss to a composite pressure vessel without relying on an adhesive layer positioned between the boss and the vessel.
It would also be an advancement in the art to provide such a method and device which prevent the boss from rotating about the pressure chamber orifice with respect to the vessel liner.
It would be an additional advancement to provide such a method and device which achieve a robust and low-cost attachment of the boss and the liner.
Such a method and device are disclosed and claimed herein.